1. Field of the Invention
The present invention relates to an electro-optic sampling probe (or prober) in which an electric field generated by a target signal to be measured is applied to an electro-optic crystal, and an optical pulse signal generated based on a timing signal is incident onto the electro-optic crystal, and the waveform of the target signal is observed according to the polarization state of the incident optical pulse signal. In particular, the present invention relates to a technique for improving the S/N ratio of the probe.
This application is based on Patent Application No. Hei 11-80543 filed in Japan, the contents of which are incorporated herein by reference.
2. Description of the Related Art
In a conventional technique, the waveform of a target signal (to be measured) can be observed by applying an electric field generated by the target signal to an electro-optic crystal; inputting a laser beam to the electro-optic crystal, and observing the waveform of the target signal according to the polarization state of the laser beam. If a pulsed laser beam is used for sampling the target signal, the measurement can be performed with a very high temporal resolution. The electro-optic sampling probe (abbreviated to xe2x80x9cE0S probexe2x80x9d, hereinbelow) employs an electro-optic probe having the above function.
In comparison with the conventional probes employing known electric probes, the above EOS probe has the following characteristics and thus has received widespread notice:
(1) A ground line is unnecessary for measuring the signal; thus, the measurement can be easily performed.
(2) The tip of the electro-optic probe is insulated from the circuit; thus, a high input impedance can be obtained and (the state of) the point to be measured is not significantly disturbed.
(3) An optical pulse signal is used; thus, wide-band measurement of a GHz order can be performed.
(4) An electro-optic crystal can be made to contact a wafer of an IC (or the like), and the laser beam can be made to converge on the wiring printed on the IC wafer, thereby enabling the measurement of thin wiring which a metallic pin cannot physically contact.
In the following explanations, the units for the optical wavelength are xe2x80x9cnmxe2x80x9d.
The structure of a conventional EOS probe will be explained with reference to FIG. 11. In FIG. 11, reference numeral 1 indicates an IC wafer connected to an external device via a power supply line and a signal line. Reference numeral 2 indicates an electro-optic element using an electro-optic crystal. Reference numeral 3 indicates an objective used for converging the beam incident on the electro-optic element 2. Reference numeral 4 indicates the (main) body of the probe, comprising a dichroic mirror 4a, a half mirror 4b, and a reflecting mirror 4c. Reference numeral 6 indicates an EOS optical system module (called xe2x80x9cEOS optical systemxe2x80x9d, hereinbelow) comprising a photodiode, a polarized beam splitter, a wave plate, and so on.
Reference numeral 7 indicates an optical fiber, to one end of which fiber collimator 7a is connected. Reference numeral 8 indicates a laser light source for supplying a laser beam to the EOS optical system. The wavelength of the outgoing laser beam having a maximum light intensity is 1550 nm. Reference numeral 9 indicates a halogen lamp for irradiating IC wafer 1 to be measured. The halogen lamp 9 may be replaced with a xenon or tungsten lamp, or the like.
Reference numeral 10 indicates an infrared camera (abbreviated to xe2x80x9cIR cameraxe2x80x9d, hereinbelow) for confirming the positioning for converging a beam onto the target wiring provided on the IC wafer 1. The image obtained by the camera is displayed on monitor 10a. The IR camera 10 has a light receiving sensitivity within a wavelength range from 500 to 1800 nm. Reference numeral 11 indicates a suction stage for fixing the IC wafer 1, which can be finely moved in the x, y, and z directions (perpendicular to each other).
FIG. 12 is a diagram showing the general structure of the EOS optical system 6. The basic structural elements of the EOS optical system 6 are a polarized beam splitter, a wave plate, and a photodiode. However, the structure as shown in FIG. 12 can reduce the noise and improve the sensitivity, thus is preferable in practical use.
As shown in FIG. 12, in the EOS optical system 6, optical path 13 is provided inside main frame 12, and half-wave plates 14 and 15, a quarter-wave plate 16, polarized beam splitters 17 and 18, and a Faraday element 19 are arranged along the optical path 13.
In addition, photodiodes 22 and 23 are provided in a manner such that they respectively face polarized beam splitters 17 and 18, as shown in FIG. 12. These photodiodes 22 and 23 are attached to the main frame 12.
Below, the optical path of the laser beam emitted from the laser light source 8 will be explained with reference to FIG. 11. In FIG. 11, the laser optical path inside the probe body 4 is indicated by reference numerals A, B, and C.
The laser beam emitted from the laser light source 8 is transmitted through optical fiber 7, and is collimated by fiber collimator 7a. This collimated beam then passes through optical path 13 in the EOS optical system 6 (see FIG. 12), and is input into the probe body 4 (refer to optical path A in FIG. 11). This input beam is deflected by 90 degrees by reflecting mirror 4c (refer to optical path B in FIG. 11). The reflecting mirror 4c used here is a total reflection mirror manufactured by depositing aluminum (or the like) on a surface of a glass material.
The laser beam deflected by 90 degrees by reflecting mirror 4c is further deflected by 90 degrees by dichroic mirror 4a (refer to optical path C in FIG. 1), and then converged by objective 3 onto a face of the electro-optic element 2 disposed on the wiring on the IC wafer 1, the face facing the IC wafer 1.
FIG. 13 shows an optical characteristic of dichroic mirror 4a. In this figure, the x-axis indicates wavelength, and the y-axis indicates transmittance. As shown in FIG. 13, the dichroic mirror 4a has the characteristic of transmitting 5% (and reflecting 95%) of a beam having a wavelength of 1550 nm. Therefore, 95% of the beam emitted from the laser source is reflected and deflected by 90 degrees.
A dielectric mirror 2a (functioning as a reflecting film) is deposited on the face (which faces the IC wafer 1) of the electro-optic element 2. The laser beam reflected by this face is again collimated by objective 3 and returns to the EOS optical system 6 through the optical paths C, B, and A (in this order). Some portions of the reflected beam are then isolated by isolator 20, and they are incident on photodiodes 22 and 23 and converted into electrical signals.
Below, the operation of measuring a target signal (to be measured) using the EOS probe having the above structure will be explained.
When a voltage is applied to the target wiring on the IC wafer 1, the corresponding electric field is applied to the electro-optic element 2, and the refractive index thereof is then changed due to the Pockels effect. As explained above, the laser beam emitted from the laser light source 8 is incident on the electro-optic element 2, and is reflected by dielectric mirror 2a and returned through the same optical path. According to the above effect, the polarization state of the beam output from the electro-optic element 2 is changed. This laser beam having a changed polarization state is again incident on the EOS optical system 6 via optical paths C, B, and A.
In the EOS optical system 6, the change of the polarization state of this incident laser beam is converted into a change of light intensity, which is detected by photodiodes 22 and 23 so as to convert them into electric signals. These electric signals are processed by a signal processing section (not shown), thereby measuring the electric signal applied to the target wiring on the IC wafer 1.
In the above conventional structure, the photodiodes 22 and 23 for detecting and outputting the change of the light intensity as electric signals are fixed to main frame 12 of the EOS optical system 6. Therefore, a change of the electromagnetic field in the IC wafer 1 or around the EOS probe, or the like, may be transmitted via the main frame to photodiodes 22 and 23 and the change may be detected as noise, thereby reducing the S/N ratio.
In consideration of the above circumstances, an object of the present invention is to provide an EOS probe for preventing the noise from being transmitted to the photodiodes and improving the measurement accuracy.
Therefore, the present invention provides an electro-optic sampling probe comprising:
a laser light source for emitting a laser beam according to a control signal from a main body of the electro-optic sampling probe;
an electro-optic element which contacts wiring on an IC wafer to be measured, and whose optical characteristics change according to the variation of an electric field generated by the variation of a voltage applied to the wiring; and
a reflecting film provided on a face of the electro-optic element, the face facing the wiring, and wherein:
the laser beam emitted from the laser light source passes through the electro-optic element, and then is reflected by the reflecting film;
the reflected beam passes through an optical path in a main frame of an optical system module wherein some portions of the reflected beam are isolated by the optical system module and are converted into electric signals; and
the optical system module comprises wavelength plates and polarized beam splitters arranged along the optical path, and photodiodes facing the polarized beam splitters, wherein each photodiode is fixed via an insulating material to the main frame of the optical system module.
Preferably, the optical system module has a wiring section for holding a lead terminal connected with the photodiode and a connector to which the lead terminal is connected; and the wiring section and the connector are insulated from the main frame.
According to the above structure, it is possible to prevent a change of the electric field around the IC wafer and EOS probe from being transmitted to the photodiode and related lead terminal and connector in the (EOS) optical system module. Therefore, such a change is not detected as noise by the photodiode, thereby improving the measurement accuracy.